A growing number of researchers have considered the acquisition of L2 grammar under incidental, or meaning-focused, learning conditions in order to understand the scope and possibilities of naturalistic adult L2 learning. However, the effect of the first language (L1) in the incidental learning of L2 syntax has not been directly studied. To address this gap, in the present dissertation, I investigated the effect of prior L1 knowledge on the implicit and explicit learning of L2 syntax under incidental learning conditions.Forty L1 English (head-initial, right-branching) and forty L1 Korean (head-final, left-branching) speakers read the novel The Mysterious Affair at Styles (Christie, 1920) rewritten in a semi-artificial language, Koreanish, which consisted of English vocabulary and head-final Korean syntax. The participants' eye movements were recorded during reading by using an EyeLink Portable Duo eye tracker (SR Research, Canada). From their eye movements, I derived two processing measures: changes in sentence reading times over time and the participants' real-time responses to word order violations. After reading, the participants were immediately tested with a surprise grammaticality judgment test (GJT) with source attributions, which was followed by the first part of the debriefing interview. Two weeks later, the participants completed a delayed GJT and the second part of the debriefing interview.Triangulation of the online and offline measures exhibited the significant and pervasive effects of prior L1 knowledge on the incidental acquisition of L2 syntax. During the exposure task (novel reading), the Korean experimental (KE) group exhibited a faster initial decrease in sentence reading times than did the English experimental (EE) group. Furthermore, only the KE group showed online grammatical sensitivity. Particularly, the Korean L1-aware participants, who later became aware of the cross-linguistic similarity between the target language and their L1, showed robust sensitivity effects, even before L1 awareness emerged. Subsequently, on the GJT, the KE group exhibited stronger evidence of implicit and explicit knowledge than did the EE group. The KE group's syntactic knowledge was significantly enhanced over time, which prompted them to outperform the EE group on the delayed GJT. These findings indicated that the Korean speakers certainly had advantages in acquiring the target word order, which followed their L1 Korean, and L1 awareness gave them an edge in knowledge development. In terms of online processing data, although both the KE and EE groups sped up over time while reading the exposure text, their learning curves had a different form. The EE group's reading data followed the power law of practice (Anderson, 1982), whereas those of the KE group did not follow a clearly discernable pattern.The findings of this dissertation suggest that L2 learners with a different L1 background perform on an unequal footing because of their prior L1 experience, especially in naturalistic learning contexts. The two L1 groups, in fact, presented opposing directions of knowledge development. The Korean participants-who had relevant prior L1 knowledge-showed a progression from implicit to explicit knowledge under incidental exposure, whereas the English participants-who lacked relevant prior L1 knowledge-began with explicit knowledge and failed to reach implicit knowledge. This dissertation therefore elucidated the multifaceted aspects of L1 influence on L2 development, uncovering the complex nature of linguistic transfer, variability in L2 development, and the moderating role of awareness in meaning-focused, incidental learning conditions.
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